Napier flood indirectly linked to atmospheric river phenomena

Two atmospheric rivers in the leadup to November 9 laid the foundation for the heavy rainfall that caused the Napier Flood. Photo / File

New research points to New Zealand's costliest floods coinciding with "atmospheric rivers", but Niwa says November's flood in Napier was not directly triggered by the phenomena.

Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that move huge columns of water vapour in the direction of preceding weather.

Not all atmospheric rivers cause damage, but most - when hitting landfall - trigger a rain event of some sort.

Niwa forecaster Ben Noll and scholar Hamish Prince, who wrote his thesis "A Climatology of New Zealand Atmospheric Rivers" last year, said although there was no atmospheric river present on November 9 when Napier experienced record-breaking rainfall, a foundation for the deluge was laid by two atmospheric rivers that passed over New Zealand in the preceding four days.

"The trigger for the event itself was more likely to be the convergence of low-level winds in the atmosphere, forcing air to rise and causing rain — when there is enough moisture around, as was the case on November 9, the rain can be intense," they said.

The research shows that Napier can expect its most frequent and impactful atmospheric river events from November to February.

Another study into atmospheric rivers in the southern hemisphere from the Australian Research Council Centre of Excellence for Climate Extremes included data from Niwa showing that nine of the 10 most expensive floods in New Zealand between 2007 and 2017 coincided with an atmospheric river event over the location where the flooding occurred.

Lead author Kim Reid told RNZ last week that although heavy rainfall is really hard to predict, weather models can pick up atmospheric rivers a lot easier since they are much bigger than an individual thunderstorm.

"So it's kind of an extra step, going from the model to the river to the rainfall is a lot easier than going from the model straight to the rainfall ... we can basically make a lot stronger prediction of extreme rainfall based on what we know about the bigger weather systems like the atmospheric rivers," she said.

In February, Hamish Prince told RNZ that his study has shown that atmospheric rivers are responsible for the vast majority of extreme rainfall in New Zealand - along with over 50 per cent of all precipitation.

"Understanding what drives these events is fundamental to planning for extreme weather and the management of freshwater resources in New Zealand."